arXiv subjects

Meng-Ru Wu

Publications and source records attributed to Meng-Ru Wu.

At least 55 records · Page 3Linked to original sources

arXiv2022

The Production of Actinides in Neutron Star Mergers

Although the multimessenger detection of the neutron star merger event GW170817 confirmed that mergers are promising sites producing the majority of nature's heavy elements via the rapid neutron-capture process ($r$-process), a number of issues related to the production of translead nuclei -- the actinides -- remain to be answered. In this short review paper, we summarize the general requirements for actinide production in $r$-process and the impact of nuclear physics inputs. We also discuss recent efforts addressing the actinide production in neutron star mergers from different perspectives, including signatures that may be probed by future kilonova and $γ$-ray observations, the abundance scattering in metal-poor stars, and constraints put by the presence of short-lived radioactive actinides in the Solar system.

astro-ph.HE↗

arXiv2022

Fast Neutrino Conversion in Hydrodynamic Simulations of Neutrino-Cooled Accretion Disks

The outflows from neutrino-cooled black-hole (BH) accretion disks formed in neutron-star mergers or cores of collapsing stars are expected to be neutron-rich enough to explain a large fraction of elements created by the rapid neutron-capture (r-) process, but their precise chemical composition remains elusive. Here, we investigate the role of fast neutrino flavor conversion, motivated by the findings of our post-processing analysis that shows evidence of electron-neutrino lepton-number (ELN) crossings deep inside the disk, hence suggesting possibly non-trivial effects due to neutrino flavor mixing. We implement a parametric, dynamically self-consistent treatment of fast conversion in time-dependent simulations and examine the impact on the disk and its outflows. By activating the, otherwise inefficient, emission of heavy-lepton neutrinos, fast conversions enhance the disk cooling rates and reduce the absorption rates of electron-type neutrinos, causing a reduction of the electron fraction in the disk by 0.03-0.06 and in the ejected material by 0.01-0.03. The r-process yields are enhanced by typically no more than a factor of two, rendering the overall impact of fast conversions modest. The kilonova is prolonged as a net result of increased lanthanide opacities and enhanced radioactive heating rates. We observe only mild sensitivity to the disk mass, the condition for the onset of flavor conversion, and to the considered cases of flavor mixing. Remarkably, parametric models of flavor mixing that conserve the lepton numbers per family result in an overall smaller impact than models invoking three-flavor equipartition, often assumed in previous works.

astro-ph.HE↗

arXiv2021

Collective fast neutrino flavor conversions in a 1D box: Initial condition and long-term evolution

We perform numerical simulations of fast collective neutrino flavor conversions in an one-dimensional box mimicking a system with the periodic boundary condition in one spatial direction and translation symmetry in the other two dimensions. We evolve the system over several thousands of the characteristic timescale (inverse of the interaction strength) with different initial $\barν_e$ to $ν_e$ number density ratios and different initial seed perturbations. We find that small scale structures are formed due to the interaction of the flavor waves. This results in nearly flavor depolarization in a certain neutrino phase space, when averaged over the entire box. Specifically, systems with initially equal number of $ν_e$ and $\barν_e$ can reach full flavor depolarization for the entire neutrino electron lepton number ($ν$ELN) angular spectra. For systems with initially unequal $ν_e$ and $\barν_e$, flavor depolarization can only be reached in one side of the $ν$ELN spectra, dictated by the net neutrino $e-x$ lepton number conservation. Quantitatively small differences depending on the initial perturbations are also found when different perturbation seeds are applied. Our numerical study here provides new insights for efforts aiming to include impact of fast flavor conversions in astrophysical simulations while calls for better analytical understanding accounting for the evolution of fast flavor conversions.

hep-ph↗

arXiv2021

Constraints on $R$-process Nucleosynthesis from $^{129}$I and $^{247}$Cm in the Early Solar System

GW170817 has confirmed binary neutron star mergers as one of the sites for rapid neutron capture ($r$) process. However, there are large theoretical and experimental uncertainties associated with the resulting nucleosynthesis calculations and additional sites may be needed to explain all the existing observations. In this regard, abundances of short-lived radioactive isotopes (SLRIs) in the early solar system (ESS), that are synthesized exclusively by $r$-process, can provide independent clues regarding the nature of $r$-process events. In this work, we study the evolution of $r$-process SLRIs $^{129}$I and $^{247}$Cm as well as the corresponding reference isotopes $^{127}$I and $^{235}$U at the Solar location. We consider up to three different sources that have distinct $^{129}$I/$^{247}$Cm production ratios corresponding to the varied $r$-process conditions in different astrophysical scenarios. In contrast to the results found by Côté et al. (2021), we find that $^{129}$I and $^{247}$Cm in the ESS do not come entirely from a single major event but get contributions from at least two more minor contributors. This has a dramatic effect on the evolution of the $^{129}$I/$^{247}$Cm ratio, such that the measured ESS value in meteorites may not correspond to that of the "$last$" major $r$-process event. Interestingly, however, we find that the $^{129}$I/$^{247}$Cm ratio, in combination with the observed $^{129}$I/$^{127}$I and $^{247}$Cm/$^{235}$U ratio in the ESS, can still provide important constraints on the properties of proposed $r$-process sources operating in the Milky Way.

astro-ph.GA↗

arXiv2021

Progress in Nuclear Astrophysics of East and Southeast Asia

Nuclear astrophysics is an interdisciplinary research field of nuclear physics and astrophysics, seeking for the answer to a question, how to understand the evolution of the Universe with the nuclear processes which we learn. We review the research activities of nuclear astrophysics in east and southeast Asia which includes astronomy, experimental and theoretical nuclear physics and astrophysics. Several hot topics such as the Li problems, critical nuclear reactions and properties in stars, properties of dense matter, r-process nucleosynthesis and $ν$-process nucleosynthesis are chosen and discussed in further details. Some future Asian facilities, together with physics perspectives, are introduced.

nucl-ex↗

arXiv2021

Supernova Constraint on Self-Interacting Dark Sector Particles

We examine the constraints on sub-GeV dark sector particles set by the proto-neutron star cooling associated with the core-collapse supernova event SN1987a. Considering explicitly a dark photon portal dark sector model, we compute the relevant interaction rates of dark photon ($A'$) and dark fermion ($χ$) with the Standard Model particles as well as their self-interaction inside the dark sector. We find that even with a small dark sector fine structure constant $α_D\ll 1$, dark sector self-interactions can easily lead to their own self-trapping. This effect strongly limits the energy luminosity carried away by dark sector particles from the supernova core and thus drastically affects the parameter space that can be constrained by SN1987a. We consider specifically two mass ratios $m_{A'}=3m_χ$ and $3m_{A'}=m_χ$ which represent scenarios where the decay of $A'$ to $χ\barχ$ is allowed or not. We show that SN1987a can only place bounds on the dark sector when $α_D\lesssim 10^{-15}$ ($10^{-7}$) for the former (latter) for $m_χ\lesssim 20$ MeV. Furthermore, this evades the supernova bounds on the widely-examined dark photon parameter space completely if $α_D\lesssim 10^{-7}$ for the former, while lifts the bounds when $α_D\lesssim 10^{-7}$ if $m_χ\lesssim 100$ MeV. Our findings thus imply that the existing supernova bounds on light dark particles can be generally evaded by a similar self-trapping mechanism. This also implies that non-standard strongly self-interacting neutrino is not consistent with the SN1987a observation. Same effects can also take place for other known stellar bounds on dark sector particles.

hep-ph↗

arXiv2021

Constraining sterile neutrinos by core-collapse supernovae with multiple detectors

The eV-scale sterile neutrino has been proposed to explain some anomalous results in experiments, \textit{such as} the deficit of reactor neutrino fluxes and the excess of $\barν_μ\to\barν_e$ in LSND. This hypothesis can be tested by future core-collapse supernova neutrino detection independently since the active-sterile mixing scheme affects the flavor conversion of neutrinos inside the supernova. In this work, we compute the predicted supernova neutrino events in future detectors -- DUNE, Hyper-K, and JUNO -- for neutrinos emitted during the neutronization burst phase when the luminosity of $ν_e$ dominates the other flavors. We find that for a supernova occurring within 10 kpc, the difference in the event numbers with and without sterile neutrinos allows to exclude the sterile neutrino hypothesis at more than $99\%$ confidence level robustly. The derived constraints on sterile neutrinos mixing parameters are comparably better than the results from cosmology and on-going or proposed reactor experiments by more than two orders of magnitude in the $\sin^22θ_{14}$-$Δm_{41}^2$ plane.

hep-ph↗

arXiv2020

Fast neutrino flavor conversion, ejecta properties, and nucleosynthesis in newly-formed hypermassive remnants of neutron-star mergers

Neutrinos emitted in the coalescence of two neutron stars affect the dynamics of the outflow ejecta and the nucleosynthesis of heavy elements. In this work, we analyze the neutrino emission properties and the conditions leading to the growth of flavor instabilities in merger remnants consisting of a hypermassive neutron star and an accretion disk during the first 10 ms after the merger. The analyses are based on hydrodynamical simulations that include a modeling of neutrino emission and absorption effects via the "Improved Leakage-Equilibration-Absorption Scheme" (ILEAS). We also examine the nucleosynthesis of the heavy elements via the rapid neutron-capture process (r-process) inside the material ejected during this phase. The dominant emission of $\barν_e$ over $ν_e$ from the merger remnant leads to favorable conditions for the occurrence of fast pairwise flavor conversions of neutrinos, independent of the chosen equation of state or the mass ratio of the binary. The nucleosynthesis outcome is very robust, ranging from the first to the third r-process peaks. In particular, more than $10^{-5}$ $M_\odot$ of strontium are produced in these early ejecta that may account for the GW170817 kilonova observation. We find that the amount of ejecta containing free neutrons after the $r$-process freeze-out, which may power early-time UV emission, is reduced by roughly a factor of 10 when compared to simulations that do not include weak interactions. Finally, the potential flavor equipartition between all neutrino flavors is mainly found to affect the nucleosynthesis outcome in the polar ejecta within $\lesssim 30^\circ$, by changing the amount of the produced iron-peak and first-peak nuclei, but it does not alter the lanthanide mass fraction therein.

astro-ph.HE↗

arXiv2020

Elastic and Inelastic Scattering of Cosmic-Rays on Sub-GeV Dark Matter

We revisit the signatures from collisions of cosmic-rays on sub-GeV dark matter (DM) in the Milky Way. In addition to the upscattered DM component that can be probed by existing DM and neutrino experiments widely discussed, we examine the associated signals in $γ$-rays and neutrinos that span a wide energy range due to the inelastic scatterings. Assuming a simple vector portal DM model for illustration, we compute both the upscattered DM flux by cosmic-ray protons, and the resulting emission of secondary $γ$-rays and high-energy neutrinos from proton excitation, hadronization, and the subsequent meson decay. We derive limits on coupling constants in the vector portal model using data from the $γ$-ray and high-energy neutrino telescopes including Fermi, H.E.S.S. and IceCube. These limits are compared to those obtained by considering the upscattered DM signals at the low-energy DM/neutrino detectors XENON1T/MiniBooNE and the IceCube. For this particular model, the limits are set predominantly by non-detection of the upscattered DM events in XENON1T, for most of the DM mass range due to the large scattering cross section at low energies. Nevertheless, our study demonstrates that the $γ$-ray and neutrino signals, traditionally considered as indirect probes for DM annihilation and decay, can also be directly used to constrain the DM--nucleon interaction in complementary to the direct search experiments.

astro-ph.HE↗

arXiv2020

Probing Cosmic-Ray Accelerated Light Dark Matter with IceCube

The direct detection of particle dark matter through its scattering with nucleons is of fundamental importance to understand the nature of DM. In this work, we propose that the high-energy neutrino detectors like IceCube can be used to uniquely probe the DM-nucleon cross-section for high-energy DM of $\sim$ PeV, up-scattered by the high-energy cosmic rays. We derive for the first time strong constraints on the DM-nucleon cross-section down to $\sim 10^{-32}$ cm$^2$ at this energy scale for sub-GeV DM candidates. Such independent probe at energy scale far exceeding other existing direct detection experiments can therefore provide useful insights complementary to other searches.

astro-ph.HE↗

arXiv2020

Neutron Star Mergers as the Main Source of R-process: Natal Kicks And Inside-Out Evolution to The Rescue

Binary neutron star mergers (BNSMs) is currently the most promising source of \textsl{r}-process thanks to the detection of GW170817. The estimated occurring frequency and the amount of mass ejected per merger indicate that BNSMs by itself can account for all the \textsl{r}-process enrichment in the Galaxy. However, the decreasing trend of [Eu/Fe] vs [Fe/H] of disk stars for [Fe/H]$\gtrsim -1$ in the solar neighborhood is inconsistent with the flat trend expected from BNSMs with a standard delay time distribution (DTD) $\propto t^{-1}$. This has led to the suggestion that either additional sources or modification to the DTD of BNSMs is required to match the observations. We investigate the effects of natal kicks received during the birth of neutron star binaries on the chemical evolution of \textsl{r}-process element Eu in the Milky Way by combining the results from the galactic dynamics code \textsc{galpy} with a one-zone Galactic chemical evolution model \textsc{omega}. We show that when key inputs from simulations of the inside-out disk evolution are combined with natal kicks, BNSMs can naturally reproduce the observed decreasing trend of [Eu/Fe] with [Fe/H] in the solar neighborhood without the need for modification to the DTD or additional \textsl{r}-process sources.

astro-ph.GA↗

arXiv2020

Fission and the r-process nucleosynthesis of translead nuclei in neutron star mergers

We study the impact of fission on the production and destruction of translead nuclei during the r-process nucleosynthesis occurring in neutron-star mergers. Abundance patterns and rates of nuclear energy production are obtained for different ejecta conditions using three sets of stellar reaction rates, one of which is based on microscopic and consistent calculations of nuclear masses, fission barriers, and collective inertias. We show that the accumulation of fissioning material during the r process can strongly affect the free neutron abundance after the r-process freeze-out. This leads to a significant impact on the abundances of heavy nuclei that undergo $α$ decay or spontaneous fission, affecting the radioactive energy production by the ejecta at timescales relevant for kilonova emission.

nucl-th↗

arXiv2020

Lifting the core-collapse supernova bounds on keV-mass sterile neutrinos

We explore the energy and entropy transport as well as the lepton number variation induced from the mixing between electron and sterile neutrinos with keV mass in the supernova core. We develop a radial- and time-dependent treatment of the sterile-electron neutrino mixing, by including ordinary matter effects, reconversions between sterile and electron antineutrinos, as well as the collisional production of sterile particles for the first time. The dynamical feedback due to the production of sterile particles on the composition and thermodynamic properties of the core only leads to major implications for the supernova physics for large mixing angles ($\sin^2 2 θ> 10^{-10}$). Our findings suggest that a self-consistent appraisal of the electron-sterile conversion physics in the supernova core would relax the bounds on the sterile neutrino mixing parameters reported in the literature for mixing angles smaller than $10^{-6}$, leaving the parameter space of the mass and mixing angles of sterile neutrinos relevant to dark matter searches unconstrained by supernovae.

astro-ph.HE↗

arXiv2020

MVIN: Learning Multiview Items for Recommendation

Researchers have begun to utilize heterogeneous knowledge graphs (KGs) as auxiliary information in recommendation systems to mitigate the cold start and sparsity issues. However, utilizing a graph neural network (GNN) to capture information in KG and further apply in RS is still problematic as it is unable to see each item's properties from multiple perspectives. To address these issues, we propose the multi-view item network (MVIN), a GNN-based recommendation model which provides superior recommendations by describing items from a unique mixed view from user and entity angles. MVIN learns item representations from both the user view and the entity view. From the user view, user-oriented modules score and aggregate features to make recommendations from a personalized perspective constructed according to KG entities which incorporates user click information. From the entity view, the mixing layer contrasts layer-wise GCN information to further obtain comprehensive features from internal entity-entity interactions in the KG. We evaluate MVIN on three real-world datasets: MovieLens-1M (ML-1M), LFM-1b 2015 (LFM-1b), and Amazon-Book (AZ-book). Results show that MVIN significantly outperforms state-of-the-art methods on these three datasets. In addition, from user-view cases, we find that MVIN indeed captures entities that attract users. Figures further illustrate that mixing layers in a heterogeneous KG plays a vital role in neighborhood information aggregation.

cs.IR↗

arXiv2020

Core-collapse Supernova Explosions Driven by the Hadron-quark Phase Transition as a Rare $r$-process Site

Supernova explosions of massive stars are one of the primary sites for the production of the elements in the universe. Up to now, stars with zero-age main-sequence masses in the range of 35--50~$M_\odot$ had mostly been representing the failed supernova explosion branch. In contrast, it has been demonstrated recently that the appearance of exotic phases of hot and dense matter, associated with a sufficiently strong phase transition from nuclear matter to the quark-gluon plasma at high baryon density, can trigger supernova explosions of such massive supergiant. Here, we present the first results obtained from an extensive nucleosynthesis analysis for material being ejected from the surface of the newly born proto-neutron star of such supernova explosions. These ejecta contain an early neutron-rich component and a late-time high-entropy neutrino-driven wind. The nucleosynthesis robustly overcomes the production of nuclei associated with the second $r$-process peak, at nuclear mass number $A\simeq 130$, and proceeds beyond the formation of the third peak ($A\simeq 195$) to the actinides. These yields may account for metal-poor star observations concerning $r$-process elements such as strontium and europium in the Galaxy at low metalicity, while the actinide yields suggests that this source may be a candidate contributing to the abundances of radioactive $^{244}$Pu measured in deep-sea sediments on Earth.

astro-ph.HE↗

arXiv2020

Neutrino signal from proto-neutron star evolution: Effects of opacities from charged-current-neutrino interactions and inverse neutron decay

We investigate the impact of charged current neutrino processes on the formation and evolution of neutrino spectra during the deleptonization of proto-neutron stars. To this end we develop the full kinematics of these reaction rates consistent with the nuclear equation of state, including weak magnetism contributions. This allows us to systematically study the impact of inelastic contributions and weak magnetism on the $ν_e$ and $\barν_e$ luminosities and average energies. Furthermore, we explore the role of the inverse neutron decay, also known as the direct Urca process, on the emitted spectra of $\barν_e$. This process is commonly considered in the cooling scenario of cold neutron stars but has so far been neglected in the evolution of hot proto-neutron stars. We find that the inverse neutron decay becomes the dominating opacity source for low-energy $\barν_e$. Accurate three-flavor Boltzmann neutrino transport enables us to relate the magnitude of neutrino fluxes and spectra to details of the treatment of weak processes. This allows us to quantify the corresponding impact on the conditions relevant for the nucleosynthesis in the neutrino-driven wind, which is ejected from the proto-neutron star surface during the deleptonization phase.

astro-ph.HE↗

arXiv2020

Neutrino Production Associated with Late Bumps in Gamma-Ray Bursts and Potential Contribution to Diffuse Flux at IceCube

IceCube has detected many TeV--PeV neutrinos, but their astrophysical origins remain largely unknown. Motivated by the observed late-time X-ray/optical bumps in some gamma-ray bursts (GRBs), we examine the correlation between IceCube neutrinos and GRBs allowing delayed neutrinos $\sim$days after the prompt gamma rays. Although we have not found any definitive correlation, up to $\sim$10\% of the events observed so far at IceCube may have been neutrinos produced by the late-time GRB activities at $\sim$1 day. Assuming a connection between some IceCube events and the late GRB bumps, we show in a model-independent way that GRB sites capable of producing late $\sim$PeV neutrinos should be nonrelativistic or mildly relativistic. We estimate the diffuse neutrino flux from such sources and find that they can possibly account for a few IceCube events. Future observations of high-energy neutrinos and late-time GRB afterglows can further test the above proposed connection.

astro-ph.HE↗

arXiv2019

Tau lepton asymmetry by sterile neutrino emission -- Moving beyond one-zone supernova models

The mixing of active neutrinos with their sterile counterparts with keV mass is known to have a potentially major impact on the energy loss from the supernova core. By relying on a set of three static hydrodynamical backgrounds mimicking the early accretion phase and the Kelvin-Helmoltz cooling phase of a supernova, we develop the first self-consistent, radial- and time-dependent treatment of sterile and tau neutrinos mixing in the dense stellar core. We follow the flavor evolution by including ordinary matter effects, collisional production of sterile neutrinos, as well as reconversions of sterile states into active ones. The dynamical feedback of the sterile neutrino production on the matter background leads to the development of a tau lepton asymmetry that grows in time until it reaches a value larger than 0.15. Our results hint towards significant implications for the supernova physics, and call for a self-consistent modeling of the sterile neutrino transport in the supernova core to constrain the mixing parameters of sterile neutrinos.

astro-ph.HE↗